Device and method for testing sensitive elements on an electronic chip

ABSTRACT

A method and device for testing an electronic chip ( 10 ) having on the surface a plurality of addressable electrically conductive sensitive elements ( 12 ) in which the sensitive elements on the chip are put in contact with a conductive solution ( 52 ), one or more sensitive elements ( 12 ) are addressed selectively in order to apply to each sensitive element addressed an electrical test signal referred to as an input signal, and a signal, referred to as the output signal, on an electrode ( 60, 60   a ) also in contact with the conductive solution, is measured.

TECHNICAL FIELD

The present invention relates to a device and method for testingsensitive elements of the surface of an electronic chip.

More and more electronic chips are being used as an analysis supportand/or an information collector in the fields of biological or chemicalanalysis, or for measuring physical quantities. The electronic chipshave for this purpose on their surface a plurality of sensors, receiversor microsystems, hereinafter referred to as “sensitive elements”. Thesensitive elements are, for example, electrodes which can be immersed ina fluid to be analyzed. These electrodes are then selectively coveredwith a lining layer sensitive to a given chemical or biologicalcompound. In some cases, the chips can also have means for selectivelyaddressing the sensitive elements in order to apply thereto anelectrical potential or for making individual electrical measurements onthese elements.

The present invention finds applications in particular in checking thecorrect functioning of electronic chips equipped with electrodes and inchecking the quality of the sensitive lining layers which cover theelectrodes.

PRIOR ART

The accompanying FIG. 1A is a schematic section of an electronic chiphaving electrodes.

The chip has a substrate 10 with a plurality of electrodes. For reasonsof clarity only four electrodes are depicted in the figure, theseelectrodes bearing respectively the references 12 a, 12 b, 12 c and 12d. A chip may, however, have a large number of electrodes. Theelectrodes have a bottom layer 13 made from a material such as titanium,for example, covered with a top layer 15 of a metal such as gold orplatinum.

In order to confer on the electrodes their function of biological orchemical sensor, they are provided with a layer of reactive material ora material able to interact with the biological material. Thesematerials may include electrically conductive polymers. By way ofexample, a modified polypyrrole can be used as a biologically sensitivelayer.

All the electrodes can be covered with the same material or can beselectively covered with different materials sensitive to differentcompounds. By way of illustration of the use of chips equipped withelectrodes, for the analysis of chemical or biological substances,reference can be made, for example, to documents (1) and (2) whosereferences are indicated at the end of the present description. Document(3), also referenced, relates more generally to the machining of siliconand the manufacture of sensors.

Returning to FIG. 1A, it can be seen that the electrodes 12 on the chip10 are electrically connected to input/output terminals 14, only one ofwhich is shown. The electrical connection is shown schematically by adot and dash line 16.

Where the chip has a small number of electrodes, each electrode can berespectively connected to a particular input/output terminal associatedwith it.

However, as shown in FIG. 1A, the connection between the electrodes andthe input/output terminals can also be effected by means of multiplexercircuits 18.

The multiplexer circuits enable addressing to take place from a smallnumber of input/output terminals to several hundreds of electrodes onthe surface of the chip.

In the sense of the present description, addressing means the putting ofat least one electrode (or other sensitive element) in electricalconnection with at least one input/output terminal. The addressing of anelectrode makes it possible not only to apply a voltage or signal to itby means of one or more input/output terminals but also to makeelectrical measurements on this electrode.

To effect addressing, that is to say to select a certain number ofelectrodes, the input/output terminals are connected to externalelectronic control devices, not shown, for example by connection means20, shown in outline.

In FIG. 1A, it can be seen that the top metal layer 15 of the electrode12 d has a porosity 23 able to allow a fluid to come into contact withthe subjacent bottom layer 13 of the electrode.

Moreover, during the manufacture of the electrodes, residues of materialmay remain on the surface of the chip. For example, residues of lacquerused during operations of forming etching masks may remain.

Such residues, marked in FIG. 1A with the references 24 a, 24 c and 24d, may partly cover an electrode, which is the case with the electrode12 a, cover it completely, which is the case with the electrode 12 c, orremain in a region with no electrodes, which is the case with theresidue 24 d.

As mentioned previously, the electrodes on a chip, intended for theanalysis of biological or chemical environments, are covered with alayer of material which confers on them their sensitivity to a givenchemical compound or biological substance.

The lining of the electrodes on the chip in FIG. 1A is illustrated byFIG. 1B. It can be seen that the electrode 12 b in FIG. 1B is coveredwith a sensitive layer 22 b, for example made of conductive polymer. Thesensitive layer 22 b covers the entire surface of the electrode. Thesensitive layer can be formed by selective deposition methods orpossibly electrochemically by selectively applying an appropriateelectrolysis current to each electrode which is to be lined.

It can be seen, on the other hand, for the electrode 12 a, that thematerial of the sensitive layer 22 a covers only part of its surface notencumbered by the residue 24 a. Thus the covering of the electrode 12 ais not homogeneous.

For the electrode 12 c, it can be seen that the residue 24 c whichcovers the entire surface prevents the formation of a sensitive layer.

Finally, it is apparent that the electrode 12 d in FIG. 1B has beenpartly destroyed during the lining step.

The bottom layer 13 of this electrode, by not being correctly protectedby the top layer 15, because of the porosity 23, has undergonedeterioration caused by the agents or fluids used for the lining of theelectrodes. Thus the electrode 12 d is unable to make analysismeasurements.

Electrodes such as the electrode 12 a or 12 c which either have nosensitive layer or are equipped with a layer which only partially coverstheir surface, lead to erroneous analysis results.

Moreover, a residue such as the residue 24 d, visible in FIGS. 1A and1B, even if it does not directly interfere with the formation of asensitive layer, risks interfering with the analysis measurements madeon the adjacent electrodes.

All the probable faults mentioned above are liable to prevent thefunctioning or interfere with the functioning of a certain number ofanalysis electrodes on a chip.

Similar problems are posed also for chips equipped with other sensitiveelements such as micromechanical elements, whose functioning may also bedisturbed by residues of material or by defects caused during themanufacturing processes.

In the field of micro-electronics, a certain number of tests designed tocheck the correct functioning of electronic chips are known.

Amongst these tests there are notably conductimetry tests and visualtests.

Conductimetry tests consist essentially of moving movable conductivespikes on the surface of the chip in order to put them in contact withconductive elements on this chip and check the passage of a measuringcurrent.

However, in an application to analysis chips, electrical tests withmovable spikes do not make it possible to detect the electrodes whichare certainly covered with a lining layer but for which the lining layerhas a lack of homogeneity or porosities, or covers only part of theelectrode, as in the case of the electrode 12 a in FIG. 1B.

In addition, electrical tests with movable spikes offer a resolutionwhich makes it difficult to use them for chips where the electrode pitchis less than 50 μm.

In addition, the movable test spikes moved on the surface of the chiprisk locally breaking a layer of residue covering an electrode andlocally coming into contact with the subjacent electrode. In this case,a measuring current may be detected and the electrode is considered tobe functional whilst it remains essentially covered by the layer ofresidue. Thus an electrode such as the electrode 12 c in FIG. 1 would bewrongly considered to be valid.

In the same way, the movement of the movable test spikes on the surfaceof a chip may mechanically damage the electrode coating layer. The testis then destructive.

Finally, it should be added that the electrical tests on the chips bymeans of movable spikes prove to be too lengthy and therefore unsuitablefor checking chips having numerous electrodes.

Visual tests, carried out by microscopy, are valid only when the coatinglayers to be checked have a certain thickness. When the electrodecoating layer is formed by a very fine deposit, such as a monomolecularlayer or a layer formed by adsorption of material on the electrodes,visual test methods prove unsatisfactory and unsuitable. This is thecase notably for chips having analysis electrodes.

Finally, the usual known tests in the field of microelectronics proveunsuitable for checking analysis chips as described previously. Inparticular, these tests do not make it possible to correctly evaluate,qualitatively and quantitatively, the functioning of the electrodes.

DISCLOSURE OF THE INVENTION

The purpose of the present invention is to propose a test method forchecking the correct functioning of a chip having sensitive elements, bymeans of a qualitative and quantitative check on the functioning of eachsensitive element on the chip.

Another aim is to propose a test method not presenting the difficultiesor limitations indicated above.

One aim is in particular to propose such a method able to be appliedsystematically to chips having addressable electrodes.

Another aim is to propose such a method which can be implemented beforeor after the formation of functional coatings on the electrodes andwhich allows not only the checking of the electrode addressing systembut also, where applicable, the quality and homogeneity of the coatinglayers on them.

Another aim of the invention is to propose an inexpensive andnon-destructive test method making it possible to automatically andindividually test, if necessary, all the sensitive elements.

One aim of the invention is also to propose a simple and inexpensivetest device intended for implementing the test method.

To achieve these aims, the object of the invention is more precisely amethod of testing an electronic chip having on the surface a pluralityof addressable conductive sensitive elements, in which the sensitiveelements on the chip are put in contact with a conductive solution, inwhich one or more sensitive elements are selectively addressed in orderto apply to each sensitive element addressed an electrical test signalreferred to as the input signal, and a signal, referred to as the outputsignal, for example on a counter-electrode in contact with theconductive solution, is measured.

The conductive solution is, for example, an aqueous or organic solutionsufficiently conductive to allow a measurable current to pass, and whichwill attack the substrate to be tested as little as possible, bothduring the electrochemical activation and at rest. It is possible touse, for example, water or a mixture of solvents optionally withsubstances such as salts added, reducing its electrical resistance.

Advantageously, the conductive solution can be a liquid identical orsimilar to a liquid which is subsequently to be analyzed or used foranalysis.

By virtue of the method of the invention, no mechanical contact with thechip is necessary, and thus the risk of damage to the sensitive elementsis removed. In addition, the method can be implemented automatically andrapidly even when the chip tested has a large number of electrodes.

According to a particular aspect of the invention, it is possible totest the sensitive elements, for example electrodes, either collectivelyor successively and individually.

By way of example, it is possible to carry out successive tests relatingto a set of sensitive elements addressed, to which a sensitive elementsupplementary to each test is respectively added.

It is also possible to perform a test relating simultaneously to aplurality of sensitive elements on the chip or to all the sensitiveelements on the chip.

According to a particular implementation of the method, the electricaltest signal can simply be an electrical potential. Electrical potentialmeans a difference in potential measured for example with respect to thecounter-electrode.

When an electrical potential is applied to an electrode or to asensitive (conductive) element, an electrical current is generatedthrough the conductive solution as far as the counter-electrode. Thismeasured current then constitutes an output signal. In the same way,when a potential is applied to several electrodes, the output signalcomprises the sum of the currents generated in the conductive solutionfrom each electrode addressed.

Optionally, the electrical potential applied to the electrodes can beadjusted with respect to a reference electrode also in contact with theconductive solution.

Another object of the invention is a device for testing addressablesensitive elements, such as the electrodes on an electronic chip. Thedevice has:

an electrochemical cell containing a conductive solution, able toreceive the addressable sensitive elements on the electronic chip,

means for selectively addressing at least one sensitive element and forapplying thereto at least one signal, referred to as the input signal,

means, connected to at least one electrode in contact with theconductive solution, for measuring at least one signal, referred to asthe output signal, in response to the input signal applied to at leastone sensitive element on the electronic chip.

In the case where the chip has a plurality of electrodes, one or more ofthese electrodes can be used for measuring the output signal. Aparticular electrode, independent of the chip, can also be put incontact with the conductive solution in order to measure the outputsignal. In the remainder of the text, the electrode used for closing thecircuit is designated as the counter-electrode.

The electrochemical cell can be formed by any device for putting thesensitive elements on the chip and the counter-electrode in contact witha conductive solution.

According to a particular embodiment, the electrochemical cell caninclude a vessel containing the conductive solution.

The counter-electrode can be separate from the chip and immersed in theconductive solution. It can also be formed directly on the chip and beput in contact with the conductive solution at the same time as theelectrodes on the chip.

The means for selectively addressing the sensitive elements and applyingan input signal thereto can include a power supply, capable ofdelivering a potential, and a system for addressing the electrodes forapplying the said potential to one or more selected electrodes.

Other characteristics and advantages of the present invention willemerge more clearly from the following description, with reference tothe figures in the accompanying drawings. This description is givenpurely as an illustration and non-limitatively.

In particular, the description which follows refers explicitly to theparticular case in which the sensitive elements on the chip areelectrodes for analysing chemical or biological substances.

To measure the output signal, the device can have a system for measuringa current generated in the electrochemical cell which passes through thecounter-electrode, as a function of the number of sensitive elementsaddressed.

However, the invention is not limited to this application. This isbecause other more complex sensitive elements such as mechanicalmicrosystems, accelerators or more generally elements having at leastone addressable conductive armature, can be tested like the simpleconductive electrodes.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A, already described, is a schematic section of an electronic chiphaving addressable conductive electrodes.

FIG. 1B, already described, is a schematic section of the electronicchip of FIG. 1A after an electrode lining operation.

FIG. 2 is a schematic representation of a test device in accordance withthe invention.

FIG. 3 is a graph showing the appearance of an output signal supplied bythe device in FIG. 2 during a test on a correctly functioning chip.

FIGS. 4, 5 and 6 are graphs depicting the appearance of an output signalof the device of FIG. 2 during tests on partially or completelydefective chips.

FIG. 7 is a graph showing the appearance of the output signal obtainedduring a non-cumulative test on a chip with 128 electrodes.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 2 shows a test device in accordance with the invention.

The device has a vessel 50 containing a conductive solution 52. In theparticular example described, the conductive solution is a decimolarsolution of LiClO₄. The conductive solution can be chosen, for example,according to the chips to be tested and notably according to thebiological or chemical environment in which the chips are subsequentlyto be used.

For particularly fragile chips, the conductive solution can be waterwith a substantially neutral pH, made conductive by adding a salt forexample.

A chip 10, having electrodes 12, is introduced into the vessel so thatthe conductive solution covers the part of the chip including theelectrodes.

The input/output terminals 14 of the chip are connected to an addressingmodule 54 by means of a connector 20.

The addressing module 54, controlled by a microcomputer 56, makes itpossible, by applying a selection signal to the input/output terminals,to select one or more electrodes on the chip in order to put them inelectrical connection with an input signal generator.

In the example described, the input signal generator is simply aconstant voltage source 58 capable of applying a potential, for example0.6 volts, to the selected electrodes.

The potential applied to the selected electrodes can be understood, forexample, as a potential difference between the electrodes and acounter-electrode 60 also immersed in the conductive solution 52.

The counter-electrode 60 can be a platinum wire.

According to a variant, shown in dot and dash lines, thecounter-electrode can also be a metallic area 60 a formed directly onthe chip 10.

The potential of the input signal can also be adjusted with respect to areference electrode 62, for example a saturated calomel electrode, whichis also in contact with the conductive solution.

An output signal from the test device reflects, for example, anelectrical current passing through the conductive solution. This currentis measured on the counter-electrode.

Measuring means 64, comprising for example a potentiostat, are connectedto the counter-electrode, possibly to the reference electrode and to theaddressing module, in order to take account of the voltage applied tothe electrodes. These means make it possible thus to record the currentin the counter-electrode according to the electrodes addressed. Thecurrent in the counter-electrode can be recorded, for example, by aplotter.

To test a chip, it is possible, for example, to successively andindividually address the electrodes one after the other by applying toeach electrode addressed the voltage delivered by the voltage source.

In this case, for each electrode the electrical current generated ismeasured and it is checked whether this current is situated in apredetermined range. When the current measured is less than the valuesof the range it can be considered, for example, that the surface of theelectrode is not homogeneous or that it is partially or completelycovered with a residue. A current greater than the values in the rangemay represent a baring of the subjacent material of the electrodesituated under the coating layer.

The test on the chip can also be a so-called cumulative test. Thepotential of the voltage source is first of all applied in a first stepto a single addressed electrode. At each following step an additionalelectrode is addressed in order to end up by applying the voltagepotential to all the electrodes.

At each step the electrical current passing through thecounter-electrode is measured. This current corresponds to the sum ofthe currents generated from each of the electrodes.

FIGS. 3 to 6 show recordings of the output signal measured in a test ofthe cumulative type for different electronic chips. These figures aregraphs indicating on the Y-axis the total current passing through thecounter-electrode, and on the X-axis a period of time. For reasons ofclarity, the graph in FIG. 3 has an intentionally expanded time scale.

The recordings in FIGS. 3 to 6 correspond to the testing of chipscomprising a matrix of 128 electrodes addressed successively andcumulatively over time, each with a potential of 0.6 volts.

In FIG. 3, two phases can be seen. During a first phase 80, an everincreasing number of electrodes is addressed until the 128 electrodesare raised to the test potential. In this phase, the current measuredincreases for each new electrode addressed.

During a second phase 82, electrodes are successively withdrawn from thegroup of electrodes being addressed. The output current measureddecreases for each electrode removed.

FIG. 3 illustrates a favourable case in which all the electrodes arefunctional and where the chip is therefore valid. This is because thegraph shows a regular incrementing (or respectively decrementing) of thecurrent during each step of connecting (or disconnecting) an additionalelectrode at the test potential. In addition, the total current remainslow, around 6 nA.

It is concluded from this not only that the addressing of the electrodesis correct, which represents a correct functioning of the multiplexingcircuit, but also that the electrode lining coating is homogeneous andwithout defect.

The graph in FIG. 4 is obtained with a chip partially covered with adeposit. The test takes place normally until the 64^(th) electrode (outof 128), marked with the reference 84. As from there the current of theoutput signal measured stabilizes and no further additional current isvisible.

The tested chip used for establishing this graph is covered over half ofits surface with a layer of lacquer used for the conditioning of thechip.

Only 50% of the electrodes on the chip are usable.

The graph in FIG. 5 is obtained with a chip having a few defectiveelectrodes.

The addressing and test take place normally up to the 51^(st) electrode.The current increments are in fact regular and the total current is lessthan 5 nA.

A current peak 86 coincides with the addressing of the 51^(st) and52^(nd) electrodes. For these defective electrodes, the output currentis greater than 100 nA.

The current peak 86 of the output signal represents a mechanicaldestruction of the (gold) surface of the electrodes concerned.

After a disconnection of these electrodes the output signal resumes anormal appearance.

The graph in FIG. 6 is obtained with a chip having a defect of theelectrochemical type.

It can be seen that the output signal corresponds to very high currents,around 1 μA or more, compared with the currents in the previous curves.It is impossible to follow the addressing on the graph.

The chip used for carrying out this experiment was previously tested bytraditional techniques in order to ensure the correct functioning of itsaddressing system.

Thus the high currents measured represent an incompatibility of theelectrode coating with the conductive solution used.

The (gold) layers covering the electrodes are not “impervious” andoxidation reduction reactions of the subjacent materials lead to adestruction of the chip.

When the tests are complete and correct functioning of the chip isverified, the chip is extracted from the conductive solution, rinsed indistilled water and then dried.

FIG. 7 is a recording carried out under the same test conditions asthose described above. However, the test, carried out on a chip carrying128 electrodes, is a non-cumulative test.

The graph indicates on the X-axis the serial numbers of the electrodessuccessively addressed, and on the Y-axis the corresponding currentpassing through the conductive solution. The current is expressed in nAand it is considered, in the example described, that a value of thecurrent of between −10 and −100 nA corresponds to a usable electrode.

DOCUMENTS CITED

(1) Biofutur N^(o) 166, April 1997, Book 91, by Jérôme Hinfray.

(2) WO-A-94 22889 (Oct. 13, 1994).

(3) “Silicon Micromachining—Sensors to Systems” by G. T. A. Kovacs etcoll., Analytical Chemistry News & Features, Jul. 1, 1996, pages407-412.

What is claimed is:
 1. A method for testing an electronic chip (10)having a plurality of conductive and electrically addressable sensitiveelements (12) intended to be tested within a conductive solution, themethod comprising: placing the sensitive elements (12) in contact withthe conductive solution (52); selectively addressing at least one of thesensitive elements (12) and applying an electrical test signal toaddress one of the sensitive elements (12); placing at least oneelectrode (60, 60 a) in contact with the conductive solution (52);measuring an output-signal at the electrode (60, 60 a); and determiningthe operational characteristics of the addressed one of the sensitiveelements (12) from the output signal, wherein said sensitive elements(12) are in an unfinished state.
 2. The method of claim 1, wherein saidaddressing further includes sequentially addressing each individual oneof said sensitive elements.
 3. The method of claim 1, wherein saidaddressing further includes sequentially addressing each subsequent oneof said sensitive elements (12) and applying the electrical test signalto each addressed one of said sensitive elements (12).
 4. The method ofclaim 1, wherein said conductive solution (52) is substantially similarto a liquid subsequently under test.
 5. The method of claim 1, whereinsaid conductive solution (52) is dissimilar to a liquid subsequentlyunder test.
 6. The method of claim 1, wherein said electrical testsignal is an electrical potential.
 7. The method of claim 6, whereinsaid electrical potential is adjusted with respect to a referenceelectrode (62) in contact with the conductive solution (52).
 8. A devicefor testing an electronic chip (10) with a plurality of addressableunfinished sensitive elements intended to be tested within a conductivesolution, the device comprising: an electrochemical cell (50) having theconductive solution (52), wherein said electrochemical cell receives theaddressable unfinished sensitive elements (12) on the electronic chip(10); means for selectively addressing at least one of the unfinishedsensitive elements (12) from the output signal and for applying to saidat least one of the unfinished sensitive elements (12) at least oneinput signal; at least one electrode (60, 60 a) in contact with theconductive solution (52); and means for detecting an output signal atthe electrode (60, 60 a), the operational characteristics of theaddressed one of the unfinished sensitive elements (12) being determinedfrom the output signal.
 9. The device of claim 8, wherein the unfinishedsensitive elements (12) are electrodes.
 10. The device of claim 8,wherein the electrode (60) is a platinum wire.
 11. The device of claim8, wherein the electrode (60 a) is provided on the electronic chip. 12.The device of claim 8, wherein said means for selectively addressing atleast one of the unfinished sensitive elements (12) from the outputsignal and for applying to said at least one of the unfinished sensitiveelements (12) at least one input signal includes a power supply (58)capable of delivering a voltage and a system (54) for addressing theunfinished sensitive elements (12) in order to apply said voltage to atleast one of said selected unfinished sensitive elements (12).
 13. Thedevice of claim 8, wherein said means for measuring an output signal atthe electrode (60, 60 a) measures a current generated in theelectrochemical cell corresponding to a number of unfinished sensitiveelements (12) addressed.